Rose works in the Ministry of Magic. She is incharge of the dementors. She goes to visit Azkaban and makes sure that the Wizarding prison is kept safe and sound, until one day, Sirius Black escapes. She doesn't know how, and she yearns to find out. Now, she was a friend of his back at Hogwarts, and she and his three friends knew he was an animagus. Though, they went their separate ways, she still kept in touch.
When Black was brought into Askaban, she kept careful watch over him, making sure he didn't die; she knew her friends wasn't a murderer. When he escaped, however, she was blamed by Fudge as much as the dementors were. Rose decided on her own to find Sirius, if only to make sure he wasn't caught.
This story begins after the fourth book, when Sirius has offered to help the Order by giving him the house.